P. carinii is an important pathogen in immunocompromised hosts, including AIDS patients. We used an animal model for establishing P. carinii infections in rats that had been immunocompromised with dexamethasone. These animals exhibited strong antibody production to a number of P. carinii proteins, most significantly to a major surface glycoprotein (MSG) of 120,000 MW. Several laboratories, including ours, have isolated cDNA clones coding for MSG. These clones code for a family of related but unique variants of MSG. Comparison of the deduced amino acid sequences derived from these clones show that the variants contain conserved regions of high similarity and other regions of little similarity. We recently demonstrated that at least four variants of MSG are expressed in an infected animal, there is a clonal pattern of expression of these variants, and this expression pattern varied with time and location, suggesting that P. carinii may shift its MSG expression to evade the host response. It remains unclear, however, if the cellular or humoral response is directed at any of the regions that exhibit sequence conservation among the variants. This information would be of value in assessing the potential of using peptides as vaccines. A peptide epitope library was constructed using the deduced amino acid sequence of a full-length MSG (GP3). Peptides of 15 amino acids in length were synthesized to span the entire 1066 amino acids of GP3. The peptides overlapped 10 amino acids at their carboxy termini. Sera from immunocompromised and normal rats showed that a number of peptides were consistently positive in enzyme-linked immunosorbent assay. To fine-tune the epitope mapping, a second peptide library was synthesized. These 13 mers were near the carboxy terminus of MSG and located in a region of the molecule that appears to be conserved among all the variants for which sequence information is available. Sera from animals immunized with recombinant GP3 elicited positive enzyme-linked immunosorbent assay to these same peptides in addition to several peptides located even closer to the carboxy terminus.